The invention relates generally to a steering device for self-propelled working machines and more particularly to a steering device which includes a steering cylinder associated with a pair of wheels, a valve unit for controlling fluid flow to the cylinders, and a hydraulic pump which supplies fluid to the valve unit via hydraulic lines. The term xe2x80x9cself-propelled working machinesxe2x80x9d is intended to include, inter alia, agricultural machines having steerable wheels, including combines and foragers, as well as tractors and other towing equipment.
A steering device for self-propelled working machines is known from EP 0 439 370 B1. This includes a steering control unit which comprises a number of electromagnetically actuatable directional control valves for setting up differing steering arrangements, for example, front wheel and all-wheel steering. A first control valve provides for switching between front wheel and all-wheel steering. A second control valve connected to the output of the first one permits two different types of all-wheel steering to be set. The steering control unit is connected to a hydraulic pump for making the fluid available. However, a disadvantage of the known steering device is that no measures are taken to maintain the maneuverability of the working machine should the power fail, the pump be defective, or some other type of defect occur.
For the purposes of ensuring that a working machine remains maneuverable in an emergency, i.e. in the event of damage, a steering device for self-propelled working machines is known from WO 96/01760 A1 wherein a steering control unit is connected to two hydraulic pumps. The fluid being conveyed from the respective hydraulic pumps is fed into a common hydraulic line before being supplied to the steering control unit. The hydraulic pumps are of different design such that, in normal operation, a first hydraulic pump having a higher throughput will initially be used, while the second hydraulic pump will only be switched in as a booster pump when the first hydraulic pump has reached its maximum possible throughput rate. If one of the pumps should fail, delivery of the fluid is continued by the other pump. Non-return valves built into the hydraulic lines prevent the fluid from flowing back into any damaged branch of the fluid-conveying line system. A disadvantage of this known steering device is that only damage occurring in a region of the fluid-conveying lines close to a respective hydraulic pump can be detected and thereby allow steering to be maintained.
An object of the invention is overcome one or more of the above-described disadvantages.
Another object is to develop a steering device for self-propelled working machines so that the working machine can be steered at any time.
In accordance with the present invention, a steering is provided, which includes at least one steering cylinder associated with at least one pair of wheels, a fluid supplied from a valve unit to all cylinders, and at least one hydraulic pump connected to the valve unit via hydraulic lines. The valve unit includes a respective switching unit incorporating at least one on-off valve associated with each steering cylinder. The on-off valves are automatically switchable in the event of a fault in a manner such that the steering cylinders may be controlled by a single steering control unit.
The special advantage of the invention lies in that the on-off valves of the switching units are designed such that they will automatically switch the steering device into a position in which the steering cylinders are connected to just a single one of the steering control units should damage occur, i.e. in the event of a fault. Preferably, a first switching unit is put into a blocking position in the event of a fault, while the on-off valves of a second switching unit are connected in a manner such that the fluid will be supplied to the steering cylinders in succession.
In accordance with one embodiment of the invention, the switching unit is arranged between the steering control unit and the steering cylinder associated therewith. Hence, should damage occur, it is ensured that the system will switch to a state wherein one of the hydraulic circuits together with any defective components therein can be completely switched out of service.
In another embodiment of the invention, the switching unit connected to the steering cylinders is connected, in the event of a fault, to a steering control unit comprising merely a hydraulic device so that a backup state will be obtained should damage occur. In this case, the steering cylinders are switched to an all-wheel steering mode by means of the on-off valves in the switching unit.
In accordance with one embodiment of the invention, the steering device consists of two hydraulic circuits of which one is associated with the steering cylinder of a hydraulic steering control unit for the front axle in the direction of travel while the other one is associated with the steering cylinder of an electro-hydraulic steering control unit for the back axle in the direction of travel. As a result of this arrangement, the steering cylinder associated with the back axle can be electrically controlled in a simple manner in correspondence with a steering program for the steering cylinder associated with the front axle. Due to the parallel control of the steering cylinders and the fact that one steering control unit can be controlled electrically, several forms, or types of steering can be implemented in a variable manner. In the event of a fault caused by a power failure, a defective pump or a defective control valve or any other sort of defect, steerage can be maintained by means of an all-wheel steering arrangement fed from a back-up pump by switching over to the hydraulic steering control unit.
In one embodiment of the invention, the on-off valves of the switching units are designed in the form of directly operated seat valves in a manner such that they can be moved into a predetermined position pertaining to the default state, without the supply of electrical energy.
In accordance with one embodiment of the invention, the on-off valves of the first and second switching unit can be switched together in a manner such that both the first steering cylinder and the second steering cylinder can be associated with the front axle or the back axle of the working machine. This arrangement permits the front axle to be controlled by a hydraulic valve so as to provide a preferred steering value, whereby the back axle will follow a certain steering program in dependence on this preferred steering value.
In a further embodiment of the invention, the emergency steering pump is in the form of a fixed displacement pump which is driven in dependence on the speed of travel of the working machine. This thereby enables the emergency steering pump to be of very simple construction.
In one embodiment of the invention, the steering cylinders are each controlled by an electromagnetically actuatable control valve, while a hydraulic device in the form of a preferred steering-angle generating device, which is operatively connected to a steering wheel, controls one or two of these control valves in accordance with a steering program. A steering characteristic corresponding to the purely hydraulic control of the steering cylinder i.e. a way of allowing the operator to have a feel for the steering is thereby produced.
Other advantages of the invention are apparent from the further dependent claims.